Inkjet recording is a method of recording a letter or an image (hereinafter sometimes simply referred to as an “image”) on the surface of a recording medium by ejecting an ink composition in the form of small liquid droplets from fine nozzles. Examples of the inkjet recording system used in practice include a method of converting an electrical signal into a mechanical signal by the use of an electrostrictive element and intermittently ejecting an ink composition stored in the nozzle head part, thereby recording a letter or an image on the surface of a recording medium, and a method of rapidly heating an ink composition stored in the nozzle head part at the portion very close to the ejection part to generate a bubble and intermittently ejecting the ink composition due to volume expansion caused by the bubble, thereby recording a letter or an image on the surface of a recording medium.
From the aspect of safety and printing property, the ink composition for inkjet recording is generally produced by dissolving various types of dyes in water, an organic solvent or a mixed solution thereof and as for various properties of the ink composition, strict conditions are required as compared with the ink composition for a writing tool such as fountain pen and ballpoint pen.
In particular, an inkjet printer is recently starting to be employed in the preparation of printed matter for advertisement and at the time of forming a color image by using a plurality of ink compositions, severer requirements are made.
This is because in a color image formed by using a plurality of ink compositions, if there is even one color having poor color hue, the color hue impairs the color balance of the image as a whole and a high-quality image can be hardly obtained.
Particularly, in the case of an ink composition using a metal phthalocyanine-based dye as the coloring material, a phenomenon that the portion of high-duty printing such as solid printing (fill printing of 100% duty) looks reddish (hereinafter referred to as a “bronzing phenomenon”) sometimes occurs. This gives rise to non-uniform color balance as the entire image and reduction in the image quality, and improvements are demanded.
Furthermore, many gloss paper sheets are recently employed as the recording medium having a texture close to the photographic touch, but due to a bronzing phenomenon caused by a specific color, the gloss feeling on the printed matter surface fluctuates to seriously impair the texture of image. Therefore, also from the standpoint of maintaining the gloss feeling of the entire image, improvements are keenly demanded.
The printed matter prepared by using the above-described ink composition is disposed not only indoors of course but also outdoors sometimes and exposed to various lights (including sunlight) and outside airs (e.g., ozone, nitrogen oxide, sulfur oxide), and attempts are being made to develop an ink composition excellent in the fastness to light and gas, but the bronzing phenomenon is outstanding particularly in the case of using a cyan ink composition improved in the gas fastness. In this respect, improvements are demanded.
In the case where a dye having poor solubility in water is used in the ink composition for inkjet recording, a so-called bronzing phenomenon is known to occur when printing is performed on a recording medium having strong acidity.
In order to solve this problem, addition of alcohol amines as a pH maintaining agent or elevation of penetrating force into paper is generally employed. The bronzing phenomenon is considered to fundamentally occur due to crystallization of a dye resulting from drying of the ink-composition-on the paper surface, and the above-described methods are effective from the standpoint of elevating the solubility to prevent the crystallization or attaining penetration into paper before the crystallization proceeds. However, in the former, the pH readily elevates to the vicinity of 11 by the addition of alcohol amines and this may cause corrosion of nozzles of the printer or may adversely affect the human body. In the latter, if the penetrability is excessively intensified, a so-called bleeding phenomenon and other adverse effects on the printing quality may be brought about. Therefore, these methods are not almighty measures.
As an alternative to these ink compositions, an ink composition where 2-oxo-oxazolidone or 1,3-bis(β-hydroxy-ethyl)urea is added for solving those problems has been proposed (see, for example, patent document 1).
Also, an ink composition where a basic amino acid is added with an attempt to solve those problems has been proposed (see, for example, patent document 2).
However, it is regrettably found from the results of various investigations and studies by the present inventors that even if these techniques are employed, in the case of an ink composition using a cyan dye excellent in the fastness (light fastness, gas fastness) of printed matter, which is intended to attain in the present invention, there is obtained no effect on the elimination or reduction of a bronzing phenomenon.
Patent Document 1: JP 6-25575 A
Patent Document 2: JP 7-228810 A